SERRA SAN BRUNO, Italy (Reuters) - Tour de France champion Bradley Wiggins has yet to show the form of last season that would make him top favorite for the Giro d'Italia, according to Juan Jose Cobo, the last non-Team Sky rider to beat the Briton on a Grand Tour.

"He's not necessarily the strongest rider," Spaniard Cobo told Reuters before Tuesday's fourth stage where Wiggins lost 17 seconds as the peloton split in the final kilometers.

Wiggins dropped from second to sixth overall on Tuesday, 34 seconds adrift of Italian Luca Paolini. Nibali is in fourth and last year's winner Ryder Hesjedal of Canada fifth, 31 and 34 seconds off the pace respectively.

Cobo, who toppled Wiggins and his Sky team mate Chris Froome to win the 2011 Tour of Spain, believes Wiggins may not be at his best, while others have shown they are in great shape.

"If Bradley had the same level as in last year's Tour then he's probably unbeatable but that's not at all clear yet this season," he said.

"Nibali has been very strong in all the races he's done this year and Wiggins hasn't yet had a chance to show his hand.

"Hesjedal has been very active in a lot of races too, and yesterday (Monday) when he attacked on that last climb, he showed he's got really good form," he added.

"That was very impressive, and he's the most recent winner of the Giro. He's somebody who has to count."

Wiggins has not won a race this season, while Nibali has taken the Tirreno-Adriatico stage race ahead of Froome and the Giro del Trentino, Italy's key warm-up race for the Giro d'Italia, ahead of Wiggins.

"Bradley is a favorite, but one of several, not the top candidate," Cobo said. "He's not been up there yet, but we'll see what happens."

STRONGEST TEAM

Cobo, who raced against Wiggins in last year's Tour de France but as part of his build-up for the Tour of Spain, his big target of the year, said Sky were the strongest team collectively in the Giro.

"When it comes to the third week, they'll be the ones who really matter, they'll make the running," he said.

Cobo said although he beat Wiggins in the 2011 Tour of Spain, wrenching the lead from him on the Angliru, Spain's toughest single climb, the Briton had not been in his top condition then.

"He was coming back from a lot of injuries and a bad crash in the Tour de France. It's nice to have done it, I won't forget that I beat him but you have to keep it in perspective," he said.

Racing injured after he crashed and hit his knee in the reconnaissance of Sunday's team time trial, Cobo was dropped on Tuesday's final climb of the Croce Ferrata early on.

"I was hoping to do well, perhaps even a top-10 result but with this injury, that's not going to happen," he said.

"My best option now is a stage win and to work for (Spanish Movistar overall contender) Benat Intxausti (who lies third behind Rigoberto Uran and Paolini). I'm not going to be beating Wiggins in this year's race, that's for sure."